The invention relates to thyristor installations for the supply of electrical energy under low voltage and very high current to a load consisting for instance of a rectifying unit and a melting metal cell. In such installations thyristors are used to control the power delivered to the load. Such thyristors are used in the primary circuit of a power transformer having the low voltage load in the secondary. Two paired antiparallel thyristors are generally associated to control the power output in each alternance to the primary side of the transformer by controlling the firing angle of conduction of such bilateral switch.
It is common practice to place a fuse in series with a static switch so that in case of excessive load the fuse will clear, thereby to protect the overall circuit. For instance, it is known from U.S. Pat. No. 3,466,527 of Chun that, when the transformer saturates, a surge of current is drawn from the input source, in which case the surge activates the fuse to disconnect and protect the circuit.
It is also known from U.S. Pat. No. 3,611,330 of R. A. Colossa to anticipate saturation of a saturable core used in an inverter circuit in order to switch off the application of power to the primary windings thereby to afford an early prevention of any rapidly increasing short circuit currents. The preventative action, there, results from the detection of impending saturation.
The present invention still relies on conventional protection in the event of an excessive current, by fuse for instance, but the concern here is with preventing saturation of the primary of the transformer rather than sensing its first manifestation thereby to provide a real fault anticipation scheme by detecting the cause of saturation.
The invention stems from the realization that saturation of the primary of the transformer is the result of a repeated discrepancy in the normally symmetrical and alternating cycling of the magnetic hysteresis characteristic of the magnetic core of the transformer. On the other hand it is observed that, if such a discrepancy does exist, it is likely to be attributable to an anomalous conduction of only one of the thyristors in the bilateral switch inserted in series configuration with the primary of the transformer. Since such a discrepancy is unlikely to occur on two sides of the bilateral switch at the same time, it is reasonable to rely on the detection of an asymmetry in the mode of operation of the primary of the transformer as an indication of defective conduction in one of the paired switches.
An object of the present invention is to provide fault anticipation on any of the controlled static switches mounted in the primary side of a high current transformer power supply.
Another object of the invention is to enable prompt disconnection of a high current transformer power supply controlled by static switches connected in the primary side by detecting a fault before it saturates the transformer.